Pollutant formation in the combustion of fossil and biogenic fuels is a problem yet to be solved. In the ideal case of complete combustion, hydrocarbon compounds CxHy are converted to carbon dioxide CO2 and water vapor H2O. Under real conditions, depending on the temperatures and pressures existing in the combustion chamber, fuel molecules and air, which consists for the most part of nitrogen and oxygen, form soot particles, nitrogen oxides, uncombusted hydrocarbons and carbon monoxide. Some of these emissions can be reduced by varying the internal engine parameters (exhaust gas recycling, injection time, duration and pressure, combustion chamber geometry, etc.). All internal engine measures for the reduction of pollutant emission result in a conflict of aims, which is referred to as the soot-NOx trade-off. When the combustion temperature is lowered, a lower level of nitrogen oxides is formed. In contrast, an increased amount of soot forms at low temperatures during combustion. In order to fulfill the exhaust gas standards for diesel engines, exhaust gas after-treatment technologies are additionally required, for example, SCR catalyst and diesel particulate filter. If water is added to the combustion process, the emissions of the nitrogen oxides and of the soot are reduced at the same time, and the soot-NOx trade-off is eliminated. Due to the high enthalpy of vaporization of water and the resulting lower combustion temperature, the formation of thermal NO, called Zeldovich NO, is reduced (Warnatz, J. et al., Verbrennung, Springer, Berlin (2001)). Since thermal NO makes up the greatest proportion of the nitrogen oxides in percentage terms, water addition can minimize nitrogen oxide emissions. At high temperatures (>2000 K), water molecules in the combustion chamber form considerable concentrations of free radicals (.OH, .O, .O2H) which, in the free-radical chain branching reaction, accelerate the degradation of hydrocarbon chains (Warnatz, J. et al., Verbrennung, Springer, Berlin (2001)).
The water can be supplied by various routes, for example, in the form of direct injection into the combustion chamber (Bedford, F. et al., SAE Technical Papers, 2000-01-2938 (2000)). At FH Trier (Trier University of Applied Sciences), a method of stratified injection of the water has been developed in which water is added through an additional channel alongside the fuel line in the injection nozzle. As a result of the static pressure drop on opening of the needle during the injection operation, the water is placed immediately before the nozzle orifice, and thus diesel and water are injected alternately into the combustion chamber. Exhaust gas analyses show that the NO emissions can be reduced by more than 50% with a water content of 40% by volume according to the load state. Both the soot emissions and fuel consumption remain unchanged (Dorksen, H. et al., Vorgelagerte Kraftstoffinenge bei geschichteter Diesel-Wasser-Einspritzung, MTZ (2007); Simon, C. and Pauls, R., Einfluβ der geschichteten Wassereinspritzung auf das Abgas-und Verbrauchsverhalten eines Dieselmotors mit Direkteinspritzung, MTZ (2004)). The water injection into the charge air in spray form causes a moderate lowering of NOx emission (at 30% water, NO lowering approximately 20%), but also a slight rise in the smoke number. The water can also be introduced into the combustion chamber in the form of a water-containing fuel. Such fuels are already available as water-diesel emulsions (Schmelzle, P. et al., Aquazole: An Original Emulsified Water-Diesel Fuel for Heavy-Duty Applications, SAE Technical Papers, 2000-01-1861 (2000) or Mikroemulsionen (Nawrath, A. et al., Mikroemulsionen und deren Verwendung als Kraftstoff, DE10334897A1 (2003)). The use of the ready-mixed water-containing fuels relies on a constant water content, which can lead to worsened ignition performance when the engine is cold-started. In the low-load range, there is additionally an observation of increased emission of the uncombusted hydrocarbons and, at high loads, only a slight lowering in the soot and nitrogen oxides. It follows from our studies that the water content be varies as a function of the operating state of the engine in order that pollutant emissions are efficiently lowered and the specific fuel consumption reduced. In dynamically operated internal combustion engines, the change in load is so rapid that a newly-defined water-fuel mixture should be provided within a few crankshaft rotations. A further point relates to the quality of the mixture. The coarser the distribution of the water-fuel domains, the more soot is produced. Therefore, a further requirement is, within a very short time and with a minimum level of energy expenditure, to achieve ultrafine distribution of the water in the fuel.
As early as 1979, Feuerman formulated an emulsion of gasoline, water and nonionic surfactants, and thus achieved a reduction in the level of environmentally harmful exhaust gases on combustion (Feuerman, A. I., Gasoline-water emulsion, U.S. Pat. No. 4,158,551 (1979)). Boehmke (Bayer A G) in 1980 formulated opalescent gasoline and diesel emulsions which were stable up to temperatures of T=−15° C. and comprised carboxamides formed from biogenic and synthetic fatty acids and alcohols (Boehmke, G., Motor fuels and furnace oils, preparation thereof and their application, (DE), B.A., EP0012292 (1980)); two years later, in 1982, there followed fuel emulsions comprising a nonionic emulsifier formed from an addition product of ethylene oxide or propylene oxide onto a carboxamide having 9-21 carbon atoms in the hydrophobic chain (Boehmke, G., Aqueous Hydrocarbon Fuel Containing Alkylene Oxide-Carboxylic Acid Amide Emulsifier, AG, B., CA1137751 (1982)). Alliger in 1981 described a method for production of emulsifiers with bunker oil (Alliger, H., Emulsified fuel oil and method of production, U.S. Pat. No. 4,244,702 (1981)), which finds use as a fuel in shipping. Lubrizol marketed the diesel emulsion fuel PuriNOx with 10 to 20% water for improvement of pollutant and energy balance (Matheaus, A. C. et al., Society of Automotive Engineers, PT-111 (Alternative Diesel Fuels): p. 1-11 (2004)). The company claims reduction in NOx emissions by 20 to 30% and in soot particulates by 50 to 65%. Bock et al. in 1992 mixed various fuels with water, short-chain and medium-chain alcohols with surfactant combinations of hydrophilic and hydrophobic surfactants (Bock, J., Robbins et al., Microemulsion Diesel Fuel Compositions and Method of Use, CA2048906 (1992)). With a quite different approach, Gunnerman in 1998 developed biphasic fuels with high water contents between 20 and 80% (Gunnerman, R., Aqueous Fuel for Internal Combustion Engine and Method of Preparing Same, MX9604555 (1998)), which were combusted in a technically complex manner in what are called “rotary engines”. The advantage of these oil-in-water (o/w) emulsions lies in a low surfactant requirement below 1%, and in the lack of ignitability outside the combustion chamber. In spite of the technical difficulty of implementation, this fuel was used to operate buses in the USA. Genova et al. in 1992 stabilized diesel fuels with relatively low water contents by means of glycolipids and large proportions of medium-chain alcohols as cosurfactants (Genova, C. et al., Hybrid Diesel Fuel Composition, (IT), E.S., U.S. Pat. No. 5,104,418 (1992)). Subsequently, in 1993, they extended these water-containing diesel fuels to include further fuel types (Genova, C. and Pappa, R., Hybrid liquid fuel composition in aqueous microemulsion form, (IT), E.S., U.S. Pat. No. 5,259,851 (1993)). Aslachanov et al. (Aslachanov, A. A. et al., Fuel for internal combustion engines, (DE), A.O.W.H.G., DE4307943 (1994)) in 1993 produced stable, low-viscosity, highly dispersed (0.1 μm) emulsions of gasoline or diesel fuel with only one surfactant: quaternary ammonium salt of fatty acid amide (C21+C30). Test bed trials with gasoline emulsions showed a 5% improvement in economic viability, reduction in carbon monoxide emissions and nitrogen oxide emissions, and a higher octane number of the new fuel. Lubrizol in 2002 developed, on the basis of amine-neutralized acylation reagents and nonionic surfactants, stabilized emulsions with ammonium nitrate as a cetane number improver (Daly, D. T. et al., Emulsified water-blended fuel compositions, US2002129541 (2002)). David in 2002 described clear stable emulsions comprising emulsifier mixtures of alcohol ethoxylates, polyisobutylsuccinimides, sorbitan esters, amine ethoxylates, fatty acid amines with addition of ethylene glycol and butoxyethanol (Martin, D. W., Compositions and a method for their preparation, US 2003/134755 (2003)). Jakush et al. in 2004 formulated, for Clean Fuels Technology INC (USA), highly stable inverse water-diesel emulsions which were sold in Australia by Shell AG (Jakush, E. A. et al., Stabile invert fuel emulsion compositions and method of making, (US), C.F.T.I., US2004255509 (2004)). Van de Berg et al. in 2007 synthesized polymers based on the ester of succinic acid, which were supposed to find use inter alia as stabilizers for formulation of fuel emulsions (kerosene, gasoline, diesel, heating oil, RME). The polymer was used in combination with emulsifiers and stabilizing components. Biocides and short-chain alcohols were added to the aqueous phase (Van de Berg, A. et al., Surface-Active Polymer and its Use in a Water-in-Oil Emulsion, in WO 2007/063036 A22007, WO 2007/063036 A2). Microemulsions of gasoline, kerosene, diesel and heating oil with water-soluble octane number-improving and freezing point-depressing additives and surfactant mixtures formed from salts of fatty acids and polyalkanolamines, and also nonionic polyoxyalkylates with nonylphenols, fatty acid amides and sorbitan esters, were patented in 1971 by McCoy et al. (McCoy, F. and Eckert, G., Process of Preparing Novel Microemulsions, U.S. Pat. No. 3,876,391 (1975)). Bourrel et al. in 1982 developed fuels with low water and high alcohol contents (Bourrel, M. et al., Microemulsion of water in a liquid fuel, Elf, A. F., U.S. Pat. No. 4,465,494 (1982)), in which the ELF Aquitaine group was involved. The group later tested water-containing diesel emulsions in bus and truck fleets in several French cities such as Paris, Lyons and Chambery, and then in Berlin, and developed the Aquazole product for heavy goods vehicles to a market-ready state by 1999 (Schmelzle, P. et al., Aquazole: An Original Emulsified Water-Diesel Fuel for Heavy-Duty Applications, SAE Papers, 2000-01-1861 (2000)). Schwab in 1984 developed low-temperature-stable water-diesel microemulsions comprising a surfactant component formed from dimethylethanolamine and long-chain fatty acids (Schwab, A., Diesel fuel-aqueous alcohol microemulsions, (US), U.A., U.S. Pat. No. 4,451,265 (1984)), and in 1984 and 1985 alcohol- and water-containing vegetable oil microemulsions of different composition for engine combustion (Schwab, A. and Pryde, E., Microemulsions from vegetable oil and aqueous alcohol with trialkylamine surfactant as alternative fuel for diesel engines, (US), U.A., U.S. Pat. No. 4,451,267 (1984); Schwab, A. and Pryde, E., Microemulsions from vegetable oil and aqueous alcohol with 1-butanol surfactant as alternative fuel for diesel engines, (US), U.A., U.S. Pat. No. 4,526,586 (1985); Schwab, A. and Pryde, E., Microemulsions from vegetable oil and lower alcohol with octanol surfactant as alternative fuel for diesel engines, (US), U.A., U.S. Pat. No. 4,557,734 (1985)). Hazbun et al. as early as 1986 developed fuel microemulsions with tert-butyl alcohol as the main component, small water contents of up to 7% by weight and methanol, using surfactant mixtures of ionic and nonionic surfactants (Hazbun, E. A. et al., Microemulsion fuel system, U.S. Pat. No. 4,744,796 (1988)). In 1986, Davis et al. filed a patent for clear, stable gasoline-based solutions comprising alcohols, nonylphenol ethoxylates and water with an improved octane number (Davis, M. and Sung, R., Clear Stable Gasoline-Alcohol-Water Motor Fuel Composition, Texaco Inc., W.P., N.Y., U.S. Pat. No. 4,599,088 (1986)). In New Zealand, Wenzel in 2003 developed combustion-improving microemulsion compositions with ionic surfactants formed from carboxylic acids, neutralized with ammonia or urea, and alcohol (Wenzel, D., Composition as an additive to create clear stable solutions and microemulsions with a combustible liquid fuel to improve combustion, NZ506262 (2003)). The solubilization of alcohols with residual water, with a water content in the overall mixture not exceeding 1.2% by weight, in the form of microemulsions was the primary aim of Akhmed (Akhmed, I., Composition of Diesel Fuel, RU2217479 (2003)) and Lif (Lif, A., A Microemulsion Fuel Containing a Hydrocarbon Fraction, Ethanol, Water and an Additive Comprising a Nitrogen-Containing Surfactant and an Alcohol, MXPA03005242 (2004)), which was achieved in the first case by addition of fatty alcohol ethoxylates and polymers, and in the second case with amine-containing surfactants. David in 2006 filed a patent for water-in-oil microemulsions comprising amphoteric surfactants, fatty acid amidoalkyl betaines, and cosurfactants such as alcohol ethoxylates, alkylamine oxides, ethoxylated fatty acid amines, which, due to the small water domain size (<0.1 μm), have inhibiting action on the growth of water organisms (Martin, D. W., Water-in-oil microemulsions, GB 2434372 (2007)).
All of the aforementioned have the aim of producing a water-containing fuel with a defined water content. The use of such a fuel in an internal combustion engine leads to inhomogeneous reduction of the pollutant emission as a function of the load state. For the efficient lowering of the soot and nitrogen oxide emissions, the water has to be metered in optimally at any operating point. This is possible only when the mixing site is positioned as close as possible to the injection nozzle. Thus, the reaction time until establishment of a new mixing ratio can be minimized. A further aspect is the distribution of the fuel and of the water in the fuel. The finer the distribution of the water, the lower the emission values.